Synchronous motor control



D. E. INMAN 1,870,456

SYNCHRONOUS MOTOR CONTROL Aug. 9, 1932.

Filed Aug. 26, 1951 /38 x39 r47 i WITNESSES.

INVENTOR ZZ Donovan E. lnman.

' ATTORNEY Patented Aug. 9 1932 UNITED STATES PATENT OFFICE DONOVAN E. INHAN, OI WILKINSBUBG, PENNSYLVANIA, ASSIGNOR '1'0 WESTINGHOUSE ELECTRIC & IANUIAGIUBIN'G COMPANY, A CORPORATION OF PENNSYLVANIA BYNGHBONOUS MOTOR CONTROL Application filed August 28 1831. Serial No. 559,410.

This invention relates generally to control system for an electric motor, and more particularly relates to a system of control for "starting a synchronousmotor and for resynchronizing a synchronous motor if pulled out of step or synchronism because of a temporar overload. One 0 the objects of this invention is to provide for starting a synchronousmotor m by applying alternating current to the stator windings, applying direct current tothe field windings a predetermined interval of time after the application of the alternating current to the stator windings, interruptin the field circuit when a temporary overload occurs on the motor, and for resynchronizing the motor and thereafter applying direct current to the field windings a predetermined interval of time after the occurrence of the temao porary overload. 1 V

One of the objects of this invention is to eflect starting of a synchronous motor during a predetermined interval of time and to eflfect resynchronizing of the motor when pulled out of synchronism by a temporary overload, during a predetermined interval o'f time smaller than the time interval required to .start' the synchronous motor. Further objects and advantages will beso come apparent from a study of the following specification when considered in connection With the accompanying drawing, the single figure of which diagrammatically shows one embodiment ofmy invention.

Referrin more particularl to the drawing, the re erencev character synchronous motor having a field winding F. A line contactor 5 is disposed to energize the stator windings of the connecting the stator windings to the supply conductors 1, 2 and 3, leading to a suitable a ternating-current generator not shown. The field winding F is provided with the usual discharge resistor 18; To efiect the excitation of the field winding, a suitable source of direct current wer, such as a batteryB, is provided, anrfghe circuit connections for the field winding are. controlled by a field contactor 31. In order that the field contactor may not chronous motor by designates a time limit re ays Y and Z are inducitlve time limit relays provided with adjustconnect the field winding F in circuit before the motor M has reached substantial synchronism, three time-limit rela s X, Y and Z are disposed to control the field contactor 31. The controlling action of these time-limit relays will be more apparent from a study of the sequence of operation durin the starting of the synchronous motor as we as from the sequence of operation for resynchronizing the motor.

The time limit relay X, while shown provided with a dashpot, may be of any type of timing device having an adjustable time constant which ma be of any value desired. The

able springs 52 and 53 for varyin the time constant, of these relays over a'prerfiitermined range.

A load responsive relay is also shown associated with supply conductor 1 leading to the stator windings, and this load-responsive relay 80 initiates theoperation of the necessary timingdevices to efiect resynchronization of the synchronous motor during operation, if pulled out of synchronism.

A better understanding of the purposes of the variouselements may be had from a discussion of the operation of the control system. Assuming that the supply conductors 1, 2 and 3 are energized by three-phase alterna'ting current and that it is desired to start the synchronous motor M, the starting pushbutton switch 4 is depressed, thereupon establishing a circuit from,the energized conductor 1 through-push-button switch 4,, the actuating coil of line contactor 5, conduc tor 6, stop push-button switch 7 and conductor 8, to the supply conductor 2. Immediately after the actuating coil of the line contactor 5 is energized, a-holding circuit is established for this coil, which circuit may, actuating coil of line contactor 5, conductor 9, and contact members 10 on the line 95 contactor 5 to the actuating coil of the line contactor. V With the operation of the line contactor 5, its contact members 12, 13 and 14 are closed and in consequence the stator windm of the motor M are supplied with energy a d? the motor M starts to accelerate. T e line contactor is also provided with contact members 19 for connecting the battery B to the conductors 21 and 26 ofthe control system, when contactor 5 is closed. It should be noted, however, that the closing of the swi h or contact members 19 does not close a circuit for the field windin F, since the contact members 54 of the eld contactor' 31 are still in open-circuit position.

Durin the initial stages of operation of the sync ronous motor, when the slip is considerable, the respective windings of the synchronous motor act like a transformer, and a current is induced in the field winding F proportional to the slip fr uency of the motor. Notto subject the fiel windmg F, and particularly the insulation in this winding, to excessive induced voltages, the field winding F is provided with a discharge resistor 18, and in consequence the current induced in the field winding during starting .flows through the resistor 18 by a circuit extending from the conductor through the back contact-members 17 of field contactor 31 and discharge resistor 18 to conductor 16.

With the actuation of the startin pushbutton switch 4, the conductors 11 an .8 were energized and the conductor 11 remains energized even after the starting push button switch is released, the energization bemg through the conductor 9 and the contact members 10 on the line contactor 5. It is, therefore, obvious that an energizing circuit is established for the 80tl18till0ll 20 of the long-time time-limit relay This relay, therefore, begins to operate, andafter a predetermined comparatively long time interval closes its contact members 44 and 46. However, as line contactor 5 is closed, the time-limit relays Z and Y are ener 'zed in part through the energization of t eir respective magnetizing coils 34 and 29. The circuit for the time-limit relay Z may be traced from the energized conductor 21, which was energized by the closing of the contact members 19 of contactor 5, through conductor 33, magnetizing coil 34 oftimelimit relay Z, conductor 35, resistor 36 and conductors 26 and 27 to the battery B. A similar circuit is established for the. magnetizing coil 29 of the time-limit relay Y, this circuit extending from the energized conductor 21 through conductor 28, actuating coil29 of time-limit relay Y, conductor 30, and resistor'32 to the energized conductor 26.

The time-limit relays Z and Y are also provided with neutralizing coils '23 and 24 respectively. These neutralizing coils are en- I ergized by a circuit extending from the ener 'zed conductor 21 through conductor 22, coil 23 of the time-limit relay Z, coil 24 of the time-limit relay Y and resistor 25 to the en-' ergiz'ed conductor 26. The neutralizing coils stants of the time-limit relays.

have but few turns so that their magnetic efiect is negligible d corresponding magnetizing. coils 34 and 29, respectively are energized. However, when the ma etizing coils are deenergized, the neutralizing coils assure that the magnetic circuits of the time-limit relays are neutralized so that the armatures of these relays will under no circumstances stick. Furthermore, the neutralizing coils, in conjunction with the magnetizing coils and the adjustable spring mechanisms 53 and 52, serve to permit accurate adjustment of the time-con- When the magnetizing coils of the time-limit relaysv Z and Y are energized, the contact members uring the timethat the 37 are immediately moved to circuit-closing position, whereas the contact members 48 Y are moved to open-circuit position.

During the initial stages of the starting of the synchronus motor M, the load-responsive device 80 will be energized. However, since-the time-limit'relay Y is immediately caused to operate, the actuatingrcoil 40 of the load-responsive device isrshunted, thereby preventing the load-responsive relay from interfering with the sequence of operation of the respective control relays.

After the s chronous motor has accelerated for a pre etermined interval of time, the contact members 46.0f the time-limit relay X close. However, the closure of these contact members cannotefiect the energization of the actuating coil of the field contactor 31 because at this instant the contact members 48 of time limit relay Z are still in opencircuit position. Since contact members 44 are alsomoved to circuit-closing the operation of the time-limit re y X, and since contact members 43 are in circuit-closing position because of the fact that the position by actuating coil 40 of the load-responsive relay is deenergized by reason of the shunt circuit through conductor 38, contact members 37 and conductor 39, the closure of contact members 44 establishes a shunt circuit for the magnetizin coil 34 of the time-limit relay Z.- This s unt circuit vmay be traced from the energized conductor 21 through conductor 42, closed contact members 43 of the load-responsive relay 80, contact members 44 of the time-limit relay X and conductor 45 to the energized conductor 35.

The magnetizing coil 34 of the time-limit relayZ is thus deenergized and, after. a predetermined interval of time, as determined by the characteristics of coils 23 and 34 and the ergized conductor 11 through conductor 47,

contact members 48 of relay Z, conductor 49, contact members 46 of relay Y, actuating coil 50 of the field contactor 31 and conductor 51 to the energized conductor 8.

. the operation of the starting button 4 or the line contactor 5.

- When the field contactor 31 has operated, a

circuit is established from the battery B through the contact members 19 of the line contactor 5, conductor 21, contact members- 54 of contactor 31, conductor 15, the field winding F of the synchronous motors M, conductor 16, field rheostat 55 and conductor 27 to the battery B.

The operation of the field contactor 31 closes contact members 56, thereby establishing a shunt circuit for the magnetizing coil 29 of the time limit relay Y. The magnetizing coil 29 is thereupon deenergized and after a predetermined interval of time, depending upon the inductive time constant of the coil 29, the electrical ,characteristicsofthe'neutralizing coil 24 and the adjustment of the spring 52,-the contact members 37 are moved to open circuit position. The main purpose of the time-limit relay Y is to prevent the opening of the contact members 87 immediately after the operation of the field contactor 31. Obviously, when the field contactor 31 is op-V erated, the synchronous motor is not or may not be completely synchronized. Therefore,-

the load-responsive relay 80, after contact members 37 open, would operate and efiect the opening of the field contactor, through the resultant action of relay Z, and the starting operation of the synchronous motor might thus be impaired. Since the time-limit relay Y prevents the operation of the load-responsive relay 80 for a short interval of time, the motor M is completely synchronized and the load-responsive device will operate only when there is an overload on the motor.

In the usual commercial applications of synchronous motors, it is well known'that overloads areof a temporary nature more often than of a long-continued nature. It is,

therefore, obvious that if the excitation of the field winding F can be removed during such temporary overload, the speed of the synchronous motor will not drop very much and the motor will'be re-synchronized, provided the field is again applied as soon as the overload is removed. By the system of co trol provided by invention, such re-syn broi V w I nization of the synchronous motor may be ve readily effected.

long-time time-limit relay X is energized, whereas the time limit relays Y and Z are deenergized, that is contact members 44 and 48 are in circuit closing position, whereas contactmembers 37 are in open-circuit position. If a temporary overload occurs, the. ac tuatin coil 40 of load-responsive device. 80, not being shunted, will be energized by such overload, thereby opening a circuit at the contact members 43. The shunt circuit for the ma etizing coil 34 of the time-limit relay Z is t us removed. The time-limit relay Z will, therefore, be ener zed, and, since this relay has a very negligi le time constant when the magnetizing coil 34 is energized,.the contact members 48 are immediately opened. Since the contact members 48 are in series circuit relation with the actuating coil 50 of the field contactor 31," the contactor will be deenergized and, in consequence, thedirect-current circuit for the field winding E will be interrupted at contact mem 54 and the field winding will be connected y contact members 17 to discharge resistor 18., The excitation of the field is thereby removed and the synchronous motor is not materially retarded during the overload. 1

With the opening of the field contactor 31, the shunt circuit forthe magnetizing coil 29 of the relay Y is interrupted at the contact members 56. time limit relay Y is thus immediately actuated to close the contact members Since the closing of shunt circuit for the coil 40, the overload relay-80 will be deenergized and a short interval thereafter the contact members 43 are placed in circuit-closing position. Since the contact members 44 of relay X are alreadv in circuit-closing position, the magnetizing coil 34.0f relay Z is deenergized and the time limit relay Z thereupon closes the contact members 48. Thecontact members 48, however, do not close immediately. The

delay is determined by the inductive time constants of :the magnetizing coil 34 and of the neutralizing coil 23 and the adjustment of the spring 53. The inductive time limit relay Z. is so designed that its time constant, when coil 34 is deenergized, may be as high as to seconds. However, by an appropriate adjustment 'of the ampere turns of the neutralizing coil 23 and the adjustment of the spring 53, a time-interval of any definite period less than 5 to" 10 seconds may be selected. It is, of course, obvious for the purposes of this invention that any time limit relay operable within any desired time may be utilized in lieu of the time-limit relay Z heretofore described, so long as the time constant selected has some reasonable relaunng normal operation of the motor, the.

the contact members) 37 reestablishes the tion to the timeof duration of a its downward movement, the actuating coil to the source of direct current overload.

After the time-limit relay Z has completed very readily be pulled back. into synchronism, and normal operation is resumed. If, during the operation of the various elements just discussed, the overload is maintained, -the cycle ofo ration for the overload relay and the tiineimit relays Y and Z is repeated, and the field winding is intermittently connected to the source of direct-current power. 0 Obviously, if the overload con- --tinues over an indefinite period, the field windings may be successively excited for a "number of times until the conventional protective overload devices usually. associated with the motor disconnect the motor from the source of supply.

While I have specifically described one embodiment of m invention, it is apparent that'the s cific e ements disclosedneed not be utilize so long as the re-synchronization terms of the appended claims.

of the motor is efiected by elements which may be interpreted to fall within the broad I claim as my invention:

1. In a control system, a synchronous mo tor, a source of alternating current, a line contactor for connecting the motor to the source of alternating current, a field winding, a source of direct-current for the field winding, 9. field contactor, a. long-time timelimit relay started by the operation of the line contactor, a short-time time-limit relay,

the operation of which is initiated upon the completion of the operation of the long-time time-limit relay, for eiiecting the operation of the field contactor, means for deenergizing the fieldcontactor, and means for initiating the operation of the short-time timelimit relay to re-s chronize the motor.

2. In a system or resynchronizin a synchronous motor that has been pulled out of step by a temporary overload, a synchronous motor, a field winding for the motor, a source of alternating-current for the motor, a source of direct-current for the field winding, a field contactor, a plurality of time-limit devices operable during starting of the motor for efiectingthe operation of v the field contactor after said time-limit devices have successively operated, and means responsive to an overload on the motor to effect the opening of said field contactor and the reclosing thereof after the successive operation of some of said time-limit devices.

3. In a system for synchronizing and resvnchronizing a synchronous motor, a synchronous motor having stator windings and in' connected time-limit devices, and means responsive to an overload on the motor for temporarily de-energizing thefield winding and for reenergizmg the field winding to resynchronize the motor, after the operation of one of said time-limit devices.

a 4. In a control system for resynchronizin asynchronous motor that has been pull out of step by a temporary overload, a synchronous motor means for energizing the motor field windings, a source of excitation for the field windings, means operable under certain operating conditions of said motor to connect said field windings to said source a predetermined interval of time after energization of said motor, disconnecting means operable under certain other operating con ditions of said motor .for disconnecting the windings from said source of excitation, and means responsive to said disconnecting means for reconnecting the field windings to said source a definite time interval less than the predetermined'interval of time.

5. In a system-for resynchronizin a synchronous motor that has been pulle out of step, stator and rotor windings, means for :energizin said windings, a pair of timelimit devices having unequal time constants operable successively to efiect energization of the rotor winding a definite time interval after energization of the stator windings, said time interval being the sum of two predetermined consecutive intervals, means for deenergizing the rotor windings and means responsive to thesmaller of the unequal time intervals for reenergizing said rotor wind- 6. In an electrical control system, a syn- I chronous motor, a source of alternating-current for the motor, means for connecting said source to the motor, a field winding forthe motor, a dischar resistor disposed to be connected to the field windin a source of direct current for the field winding, a field contactor for connecting the source of directcurrent to, and disconnecting said discharge resistor from, the field winding, a air of time-limit relays successively operab e during starting of the synchronous motor to effect'the operation of the field contactor, a relay-responsive; to a temporary overload on the motor'for de energizing the field contactor, and-a time-limit relay for controlling the operation of the field contactor to reconnect the field windings to the source of direct current -a predetermined interval of time after the occurrence of the overload.

7. In a system .for resynchronizin a synchronous motor that has been pull out of step, stator and rotor windings, a pair of contactor means for energizing said virindings, three time-limit devices havin (Fredetermined time constants two of evices being sequentially and interdependently operable to effect energization of said rotor winding a definite time interval after energization, said time interval being the sum of two predetermined consecutive intervals,

overload responsive means disposed to effect deenergization of the rotor winding by deenergizing said contactor means, means re sponsive to the deenergization of the one of said contactor means to control the operation of the third time limit device and one of said' other time limit devices to effect operation of said contactor means to reenergize the rotor winding.

8. Ina control system for a synchronous motor having stator and rotor windings, a source of alternating-current energy, a llne contactor for connecting the stator to the source of energy, a source of direct-current energy, a field contactor for connecting the rotor to the source of direct-current, a timelimit device energized by the operation of the line contactor to initiate its timing 0 eration, two other timin devices energize by the operation of the llne contactor, said two timelimit devices having a negligible time constant when energized, an overload device, switching means on one of said last named timing devices preventing the operation of said overload device during starting, switching means on the first named timing device for deenergizing one of said last named timing devices, switchin means on said last mentioned timing devices to effect the operation of the field contactor a definite time after the operation of the line contactor, said definite time being the sum of the predetermined time intervals of said two timin devices, a switch on the field 'contactor e ectingthe deenergization of the last named timelim1t device, whereby, the overload device is permitted to operate to effect deenergization of the field contactor, whereupon said field contactor is reenergized after a predetermined time interval as determined by the operation of two of said time-limit devices.

In testimony whereof, I have hereunto subscribed my name this 21st day, of August,

. DONOVAN E. mm. 

